1. Field of the Invention
The present invention relates to a resonance drive actuator for reciprocatively driving an output shaft to which a load is connected, by an electromagnetic driving force, at least in an axial direction at an axial resonance frequency that is determined by the spring force of a spring member and the mass of a movable section or in a rotational direction at a circumferential resonance frequency that is determined by the spring force and the inertia of the movable section.
2. Description of the Related Art
An actuator that can be used as a drive section of an electric shaver or an electric toothbrush reciprocatively moves a movable section in an axial direction and a rotational direction by means of an electromagnetic driving force. At this time, the movable section undergoes a reciprocative motion at an axial resonance frequency that is determined by the spring force of a spring member and the mass of the movable section and at a circumferential resonance frequency that is determined by the spring force and the inertia of the movable section.
FIG. 8 shows an example of such a conventional actuator. A movable section of the actuator includes an output shaft that outputs a driving force, and a columnar plunger formed of a magnet that is magnetized in a circumferentially divided manner. The movable section is supported so as to be reciprocatively movable in a rotational direction and in an axial direction in relation to a stationary casing, by two bearings accommodated in two corresponding bearing holder portions fixed to the stationary casing. A core that forms magnetic poles whose number is equal to or greater than that of magnetic poles of the magnet, and coils wound on the core are disposed around the outer circumferential surface of the plunger, thereby forming an electromagnetic drive section that is fixed to the inner surface of the stationary casing. The magnetic poles of the stationary electromagnetic drive section and the magnetic poles of the magnet of the movable section are arranged so as to generate a rotational driving force in the circumferential direction when current is applied to the coils, and are axially offset from each other so as to generate a driving force in the axial direction. A coil spring is disposed at one end of the output shaft between a movable spring retainer that is fixed to the output shaft, and a stationary spring retainer that is fixed to the casing.
When alternating current is applied to the coils, the electromagnetic drive section axially attracts the movable magnet (and the output shaft fixed to the movable magnet) against the restoration force of the coil spring or repulses the movable magnet in the reverse direction, while attracting the movable magnet in a predetermined circumferentially rotational direction or repulsing the movable magnet in the reverse rotational direction. The plunger thus undergoes a reciprocative motion in the axial direction and in the rotational direction.
The coil spring must be securely fixed at its opposite ends to relatively driven members, such as the movable section and the stationary section. However, securely fixing the coil spring against an axial force and a rotational force is difficult. Also, the coil spring has a problem of having a long length. Further, since installing the coil spring with high degree of coaxiality is difficult, a loss, such as the friction loss of a bearing, arises, raising a difficulty in obtaining resonance.
In order to cope with the above problem, use of a plate spring in place of the coil spring is known. FIG. 9 shows another conventional actuator that uses plate springs (refer to Japanese Patent No. 3475949). Use of the first to third plate springs facilitates a reduction in the mass of the spring members and the overall length of the actuator.
A movable section includes a columnar plunger formed of a magnetic material, such as an iron material, and an output shaft that outputs a driving force. An annular coil is fixed to the inner surface of a casing in a manner of surrounding the plunger and serves as an electromagnetic drive section. Two annular magnets are disposed on the corresponding axially opposite sides of the coil and magnetized symmetrically with respect to the coil. Two yokes are disposed on the corresponding opposite sides of each of the magnets. A first plate spring is disposed between the casing and one end of the output shaft; and a second plate spring, an amplitude control weight, and a third plate spring are disposed, in the order given, between the other end of the output shaft and the casing.
When no current is applied to the coil, the plunger remains stationary at the illustrated position where a magnetic force that the magnets impose on the plunger via the yokes balances with a spring force of the first to third plate springs. When current is applied to the coil in one direction, a magnetic force that is generated by magnetic flux generated in the plunger and magnetic flux of the magnets causes the plunger to move toward one magnet against the restoration force of the first plate spring. When current is applied to the coil in the reverse direction, the plunger moves in the reverse direction against the restoration force of the first plate spring. Thus, application of alternating current to the coil causes the plunger to undergo an axially reciprocative motion.
However, the plate springs can produce a desired spring force for axially reciprocative motion of the movable section, but fail to produce a desired spring force for rotationally driving force with which the movable section is reciprocated in the circumferential direction in relation to the stationary section. In other words, the plate springs exemplified in FIG. 9 are not designed for applications in which the movable section is reciprocated in the circumferential direction.